Reduction of gun barrel wear



Oct. 5, 1965 Filed NOV. 4, 1957 D. E. MGLENNAN 3,209,689

REDUCTION OF GUN BARREL WEAR 2 Sheets-Sheet l Oct. 5, 1965 D. E.MGLENNAN `3,299,689

v REDUCTION OF GUN BARREL WEAR Filed Nov. 4, 1957 2 Sheets-Sheet 2 Ey IWala/Va@ t United States Patent 3,209,689 REDUCTION 0F GUN BARREL WEARDonald Elmore McLennan, Carde, Quebec, Canada, assignor to I-Ier Majestythe Queen in the right of Canada as represented by the Minister ofNational Defence, Ottawa, Ontario, Canada Filed Nov. 4, 1957, Ser. No.694,453 Claims priority, application Canada, Nov. 14, 1956, 718,214Claims. (Cl. 102-38) Ever since the introduction of colloidal gunpropellants over half a century ago, the wear of gun barrels hasrepresented an important problem. In many instances the service life ofa gun barrel is only a few h-undred rounds. As a consequence, frequentcostly and inconvenient replacements of the gun barrel become necessary.Furthermore, the compensations made for wear at various stages duringthe life of a -gun barrel are of an approximate nature and are thus aserious source of inaccuracy.

The excessive wear or erosion of the surfaces of the bore of a gunbarrel is due to the high temperature of the propellent gases, which inthe case of guns such as 17 and 20 pr.s and heavier artillery is withinthe range 2000-4000 C. Gun barrels are made of steel which, due to itsiron content has the following critical temperature regions, the AC1transition region between 1340" F. and 1415 F. and the austenitic orgamma region up to the melting point region of 2650-2750 F. Theforegoing are typical values for gun barrel steel having an analysis ofcarbon 0.4%, nickel 1.65-2.0%, chromium 0.7-0.9%, and molybdenum(l5-0.3%. The firing of each round causes a thin surface layer of thebore to be raised to a temperature which exceeds the AC1 transistionpoint, and may exceed the melting zone causing localized volurne changeswhich strain and fatigue the metal. This high surface temperatureimpairs the ability of the steel to withstand the attrition due to thepassage of the projectile and the propellent gases. In addition, avariety of chemical reactions such as oxide and nitride formation andlocalized changes of carbon content causing erosion of the steel takeplace to an increased extent at high temperatures.

The high temperatures of the propellant gases which are of the order of3000 C. give rise to an additional problem in that the permissible rateof fire is limited by the temperature of the gun barrel. A high rate offire results in increased wear, since the bore surface is additionallyheated with each successive round. Exceeding the permissible rate of recan result in permanent damage to the gun or may cause a seriousaccident. A closely related problem is that of length of burst in thecase of automatic guns, since their effectiveness against quickly movingtargets such as aircraft depends on the permissible length of burst.

The foregoing problems have hitherto been accepted as being incapable ofpractical and effective solution and considerable effort has beendevoted to the improvement of the quality of gun barrel steel, designingdriving bands which will reduce and compensate for wear and towards theformulation of cool propellants having iiame temperatures lower thanthose Iof earlier propellants and therefore causing less wear andpermitting a higher rate of re. These formulations have provided merelya highly incomplete partial solution and are accompanied bydisadvantages since the use of cool propellants res-ults in a decreasein the propulsion developed by a Igiven weight of charge and sometimescauses ignition difficulties. Also, cool propellants are complex andexpensive mixtures the preparation of which requires careful blending.Irregularities in quality and performance 3,209,689 Patented Oct. 5,1965 are of frequent occurrence and are a source of inaccuracy.

In accordance with this invention a boundary layer of cool inert gasesis provided between the surface of the bore and the propellant gases.This boundary layer is formed suiciently rapidly upon the combustion ofthe charge and is of suicient thickness extent and durationsubstantially to prevent the sur-face of the bore from being raised bypropellant gases having temperatures of about 200G-4000 C. to atemperature at which a deleterious metallurgical transition occurs. Theboundary layer will be reasonably effective if it at least halves thebore surface temperature increase. It has ben found that in some casesthis reduces wear by factors of 30 or more.

In accordance with a preferred more limited aspect of this invention theboundary layer of cool inert gases is generated by a composition such asan organic high polymer, with or without the inclusion of fillers, whichis endothermically decomposed by heat from the burning of the charge andthe propellant gases. Preferably this composition has an enlargedsurface area and is in the form of a sleeve surrounding the forward endof the charge. The gas generated by the composition forms a laminarinsulating boundary layer.

In the drawings which illustrate the preferred embodiments of thisinvention:

FIGURE 1 is a sectional elevation View of a cartridge case in accordancewith this invention.

FIGURE 2 is a detailed perspective view of the sleeve used in FIGURE 1.

FIGURE 3 is a sectional elevation view showin-g part of a gun barrelduring the firing of a round of the type illustrated in FIGURE l.

FIGURE 4 is a diagram illustrating the results of tests red with a 17pr. gun.

In FIGURE 1 of the drawings, a cartridge case 10 contains a propellentcharge 11 and an igniter 12, and a projectile 13. A sleeve 14, shown indetail in FIGURE 2, of a coolant gas-forming composition encircles theforward end of the charge, extends back to about the centre of thecharge, and is secured to cartridge case 10 by a layer 15 of an adhesivesuch as a glyptal resin adhesive. Other physically and chemicallycompatible adhesives may be used. Sleeve 14 preferably includes aneckpiece 14a which is a forward extension of sleeve 14 tapered toconform with the reduced portion 16 of the cartridge case and having asimilar function to the main portion of sleeve 14.

FIGURE 3 illustrates part of the gun barrel 17 of, in this case, abreach block obturated gun containing a propellent charge 18. In FIGURE4 the projectile 13a is shown imediately after ring of the charge 18.The charge provides propellent gases 19 to propel projectile 13a. Theheat generated by the burning of charge 18 and from propellant gases 19decomposes sleeve 1411 causing the formation of a boundary layer 20 ofcool inert gases which insulate the bore surface 21 from the heat ofpropellant gases 19. Since no cartridge case is included in theembodiment shown in FIGURE 3, it is difficult satisfactorily to securesleeve 14h in position to prevent it from being discharged prematurelywith the projectile. Sleeve 14h should extend back to at least thecentre of the charge and should closely engage the surface of the boreso that the burning of the charge will tend to force the sleeve againstthe surface of the bore.

The preferred material for the sleeves 14, 14a and 14b is polyurethaneisocyanate foam which has been tested and shown to give improvements :inwear by a factor of from 2 up to 30 or more depending on the type ofammunition. However, other materials such as nylon, silk, wool,polymethyl methacrylate, polystyrene, polyethylene, silicones, siliconerubbers and natural and synthetic rubbers Jce may be used. Thepolyurethane foam may be combined with a filler such as for exampleammonium oxalate which is a gas producer so as to augment the boundarylayer, is compatible and has a high decomposition energy to create acooling effect. The ammonium oxalate in the form of a finely dividedpowder is blended with the polyurethane monomers. A suitablepolyurethane foam has a density of 25 pounds per cubic foot and has thefollowing composition:

Resin pre-polymer yParts by weight Polyethylene glycol 200 10.5Polypropylene glycol 1200 6.5 `Castor oil (Hydroxyl No. 160-165 Acid No.

1.8 max.) 36.5 2,4-toluene di-isocyanate 46.5

Resin catalyst Part per 100 parts pre-polymer Glycerine, 98% 7.5Polyethylene glycol 200 3.7 Armeen DM16D 1.0

Where a filler is being used suitable proportions of the resinpre-polymer catalyst and filler are as follows:

Parts per 100 Resin pre-polymer 58.8 Catalyst 7.2 Ammonium oxalateKmonohydrate 34.0

Almost any material which will decompose endothermically to form gasescan be used lfor sleeve 14. However,

it is preferred that an organic high polymer be employed since thesehave large numbers of chemical bonds, the breaking of which absorbs heatenergy and these products on decomposition yield adequate quantities ofcooling gas. The breakdown temperature of the material should be as lowas possible without being so low as to give rise to lack of stabilityduring storage. If the breakdown temperature is high there will be adelay in the evolution of the gas upon the combustion of the charge andthe temperature of the gas in the boundary layer will be high. Thepreferred breakdown temperature is between about 200 F. and 300 F.

The material used for the sleeves should have no adverse effects on thechemical stability of gun propellants in contact with it and shouldabsorb as little nitroglycerine as possible. `It is undesirable to usematerials which will produce toxic or corrosive products of combustionhowever fea-med polyvinyl chloride has been tested and found to reducewear by a factor of about ten in spite of the possible corrosive effectof the hydrochloric acid which is one of its decomposition products.

The lmaterial used for the sleeves should have an enlarged surface areabecause of the importance of providing immediate decomposition. The useof material in the form of foam has been found to be more effective thanthe use of material in the form of a solid sheet. However, this willdepend somewhat on the decomposition temperature of the plastic and onthe flame temperature of the propellant.

The sleeve need not extend the full length of the cartridge but shouldextend rearwardly to the centre of the charge since the boundary layeris poorly formed in the rear portion. The rear portion of the sleevewould be wasted and there would be a fouling up of the igniter if a fulllength sleeve were used. If the sleeve is located too far forwardly ofthe centre of the charge it is possible that the sleeve will be expelledfrom the gun before it has undergone appreciable decomposition. Thus inearly ring trials it was found that polyvinyl chloride foam in the formof a doughnut on top of the charge gave a reduction in the totalquantity of heat transmitted to the gun of at best only to 15%. Where acartridge case is |used, an adhesive can conveniently be used to securethe sleeve to the cartridge case. The adhesive will be effective inspite of the high temperatures as the material in the sleeve acts as athermal insulant for the adhesive and will be decomposed Vbefore theadhesive.

The invention is further illustrated by the following examples:

EXAMPLE 1 A total quantity of 3 oz. of polyurethane foam was applied inthe form of a sleeve encircling the charge and adhered within thecartridge case of a 17 pr. gun. The polyurethane foam was applied as aneighth inch thick l0 sheet of 20 lb./cu. ft. foam extending from thefront of the charge about half of the distance towards the rear of thecharge. Firing trials were conducted of guns using ammunition thusmodified in comparison with guns using conventional ammunition. The wearwas measured by star gauge both before and after a sequence of roundsand was found to give a factor of improvement of over 30.

EXAMPLE 2 The results of trials with various types of coolant are 20shown in Table 1.

Table 1-17 pr. trials CFI Rate of wear 9,.. Type ol' ammunition Numberof per round 0 rounds (1 C of R),*

inch

Cooled polyurethane, 90 gms 20 0. 0000 *l inch forward of commencementof rifling.

Additional data as to further tests is shown diagrammatically in 'FIGURE4. In FIGURE 4, pvc refers to polyvinyl chloride and iso refers topolyurethane isocyanate.

l EXAMPLE 3 40 A total quantity of about 3 oz. of polyurethane foam was:applied in the form of a sleeve encircling the charge .and adheredwithin the cartridge case of a `20 pr, gun. The polyurethane lfoam wasapplied Ias a one-sixteenth inch thick sheet of 3,0 lb./cu. ft.material, extending from lthe front of the charge about half of thedistance towards the rear of the charge. Firing trials were conducted asin Example 1. The results are set out in Table 2.

Table 2.-20 pr. trials (armour piercing discarding sabot ammunition)Rate of wear Type of ammunition Number per round of rounds (1 C of R),

inch

Many 0.0011

The invention is primarily applicable to ordnance ammunition sincebarrel time is longer than that of small arms and the duration ofexposure to the gases is, therefore, longer than in the case of smallarms. In small arms the barrel time is less but the avoidance of wear ishighly beneficial. The invention can also be applied in the case ofsmall arm-s ammunition to increase the rate of fire by avoidingoverheating. In the case of small arms ammunition, it will beparticularly important, due to the lower duration of exposure that thesleeve material have a high surface area so that it will readily bedecomposed.

The primary advantages of the invention are the saving of wear andincreasing the accuracy and rate of fire. Other advantages, however,include the possibility of using hotter propellants to give highermuzzle velocities and the possibility of design improvements for exampleto the driving bands which previously have been designed with Itheminimizing yof Wear and the necessity of compensating for wear ascontrolling factors.

We claim:

1. Ammunition comprising a projectile, an explosive charge :forgenerating propellent gases 'to propel said projectile through the boreof a gun barrel in which the ammunition is adapated to be used and asleeve of a compositon having a foam-like cellular structure providingan enlarged surface area encircling at least the forward portion of saidcharge for generating a boundary layer of cool inert gases between saidpropellent gases and the surface of said bore substantially immediatelyupon the burning of said charge and lasting for the duration of thetravel of the projectile through the bore, said laminar boundary layeracting at least to halve the bore surface temperature increment, saidcomposition comprising an organic high polymer which decomposes at atemperature between 200 and 300 F.

2. Ammunition as in claim 1 in which said organic high polymer is in theform of a solid foam.

3. Ammunition as in claim 2 in which said organic high polymer ispolyethylene isocyanate.

4.-y Ordnance ammunition comprising a cartridge case, a projectilepositioned at an open end of said cartridge case, an explosive chargeWithin said cartridge case for generating propellent gases to propelsaid projectile through a bore of a steel gun barrel in which theammunition is adapted to be used, a sleeve of a foam-like cellula-rstructure gas generating composition encircling at least the lforwardpart of .said charge, said composition decomposing upon t-he burning rofsaid charge to provide a laminar boundary layer of cool inert gasesbetween the propellent gases .and the surface of said bore substantiallyto .prevent ythe bore surface temperature from rising Ibeyond thealpha-gamma transition temperature of said steel and means for securingsaid sleeve to .the inner surface of said cartridge oase, -said gasgenerating composition being an organic high polymer which decomposes ata temperature between 200 and 300 F.

5. Ordnance ammunition as in claim 4 in which said gas generatingcomposition is in the form of a solid foam.

References Cited by the Examiner UNITED STATES PATENTS 45,227 1 1/ 64Connel 102-97 1,187,779 6/ 16 Patten. 1,842,445 l/ 32 Clyne 102-442,180,239 11/39 Holland et al. 102-43 2,188,465 1/ 40 Perrin 102.-432,559,275 7/51 Brown 102-95 2,703,529 3/ 55 Tuckerrnan et al. 102-432,820,412 l/58 Beeuwkes et al. 102-93 2,897,758 8/59 Miller et al.102-42 BENJAMIN A. BORCHELT, Primary Examiner.

SAMUEL BOYD, ARTHUR M. HORTON, SAMUEL FEINBERG, Examiners.

1. AMMUNITION COMPRISING A PROJECTILE, AN EXPLOSIVE CHARGE FORGENERATING PROPELLANT GASES TO PROPEL SAID PROJECTILE THROUGH THE BOREOF A GUN BARREL IN WHICH THE AMMUNITION IS ADAPTED TO BE USED AND ASLEEVE OF A COMPOSITION HAVING A FOAM-LIKE CELLULAR STRUCTURE PROVIDINGAN ENLARGED SURFACE AREA ENCIRLING AT LEAST THE FORWARD PORTION OF SAIDCHARGE FOR GENERATING A BOUNDARY LAYER OF COOL INERT GASES BETWEEN SAIDPROPELLANT GASES AND THE SURFACE OF SAID BORE SUBSTANTIALLY IMMEDIATELYUPON THE BURNING OF SAID CHARGE AND LASTING FOR THE DURATION OF THETRAVEL OF THE PROJECTILE THROUGH THE BORE, SAID LAMINAR BOUNDARY LAYERACTING AT LEAST TO HALVE THE BORE SURFACE TEMPERATURE INCREMENT, SAIDCOMPOSITION COMPRISING AN ORGANIC HIGH POLYMER WHICH DECOMPOSES AT ATEMPERATURE BETWEEN 200* AND 300*F.